1. Field of the Invention
The present invention relates to a pre-emphasis automatic adjusting method and a data transmission system and more particularly to the pre-emphasis automatic adjusting method effective for pre-emphasis for high-speed transmission and the data transmission system employing the method.
The present application claims priority of Japanese Patent Application No. 2006-094866 filed on Mar. 30, 2006, which is hereby incorporated by reference.
2. Description of the Related Art
Conventionally, in a signal transmission system, by necessity of high-speed transmission of signals, pre-emphasis is performed on signals to be transmitted in a transmitting circuit and the signals are transmitted to a receiving circuit. In order to optically perform pre-emphasis in a transmitting circuit, pre-emphasis strength is automatically adjusted. The conventional automatic adjustment to be made to pre-emphasis strength is as follows:
The conventional signal transmission system having a function of automatically adjusting pre-emphasis strength, as shown in FIG. 6, includes a random pattern generating section 31, an automatic adjustment controlling section 32, a selector 33, and a transmitting circuit 34 on a transmitting circuit side 30 and a Veye (eye aperture) judging section 41, a sampling clock controlling section 42, a reference potential generating section 43, and an adjustment controlling section 44 on a receiving circuit 40. In this system, pre-emphasis strength is automatically adjusted so as to set an optimum value. When the automatic adjustment is started, a signal having a random pattern is applied from the random pattern generating section 31 to the selector 33 and, when an automatic adjustment controlling signal for the start of the automatic adjustment from the automatic adjustment controlling section 32 to the selector 33, a signal having a random pattern is supplied to the transmitting circuit 34 having a differential amplifier structure and a positive signal and an opposite signal are transmitted through each transmission path to the receiving circuit 40.
A potential of the signal to be received by the Veye judging section 41 is compared with a reference potential to be applied from the reference potential generating section 43. This comparison is sequentially performed while the referential potential is being changed, in order, by the referential potential generating section 43 from its low level to its high level and while a center of an eye aperture is being controlled by a sampling clock generated by the sampling clock controlling section 42. As a result, an eye aperture (Veye) occurring between a positive phase signal and an opposite signal in a direction of a signal amplitude (voltage direction) is judged and recognized.
The above comparison of signals to be received is made from a low level signal to a high level signal and, with a progress of the comparison, pre-emphasis strength to be provided from the transmitting circuit 34 to a transmitting signal is increased sequentially by a pre-emphasis strength adjusting signal to be supplied from the adjustment controlling section 44 to the transmitting circuit 34. FIG. 7(a) is a diagram illustrating a waveform at an initial stage of the adjustment. When the Veye judging section 41 judges that the eye aperture (Veye) has become maximum, that is, the adjustment has become optimum, the adjustment controlling section 44 transmits an adjustment completion signal to the automatic adjustment controlling section 32 to complete the automatic adjustment [see FIG. 7(b)].
Moreover, technologies related to an eye-pattern are disclosed in Patent Reference 1 (Japanese Patent Application Laid-open No. Hei 07-264248), Patent Reference 2 (Japanese Patent Application Laid Open No. 2005-094172), and Patent Reference 3 (Japanese Patent Application Laid-open No. 2001-144819). The technologies are related to technologies that deal with the eye aperture in a direction of an amplitude of the eye-pattern, that is, with the eye aperture in a direction of voltage. Furthermore, pre-emphasis technology that serves to uniformalization of performance of all the optical wavelength division multiplexed signal to be transmitted via an optical transmission path is disclosed in Patent Reference 4 (Japanese Patent Application Laid-open No. Hei 8-321824).
However, the above conventional technologies have problems. That is, in all of the above technologies, it is necessary that the random pattern generating section 31 is provided on the transmitting circuit side 30 as an essential element. This presents a problem in that a circuit scale is made to be large and an increase in a circuit area required on a semiconductor substrate is unavoidable. Moreover, in order to detect a received eye aperture (Veye), a step of finely changing a reference potential for comparison and judgment is necessary and indispensable, which causes time required for the adjustment of pre-emphasis to be longer.
The technologies disclosed in the Patent References 1, 2, and 3, as described above, are related to the eye aperture in a direction of an amplitude of the eye-pattern, that is, to the eye aperture in a direction of a voltage and, therefore, have the same problems as described in the above prior art. Also, in the technology disclosed in the Patent Reference 4, an optical signal to noise ratio is used for optimal setting of the pre-emphasis, there are the similar problems as with the above other conventional technologies.